Most offshore hydrocarbon production operations carried out in deep water are conducted from floating vessels, such as a production and storage vessel. Once an offshore well begins producing hydrocarbons, it is necessary to provide for the transmission of the produced hydrocarbons from a wellhead at the ocean floor to the production and storage vessel. Typically, a plurality of flowlines from one well, or a plurality of offshore wells, are connected to a production and storage vessel via a marine riser, or riser pipe, which has associated therewith a plurality of riser flowlines.
One problem associated with such marine risers is the potential for structural failure caused by horizontal movement of the production and storage vessel from the vertical centerline from the point on the ocean floor where the submerged flowlines are connected to the marine riser. Wind, waves, and currents generally cause some vessel movement during production operations, even though the vessel is properly moored. It is therefore necessary to increase the flexibility of the large diameter, relatively stiff riser, to prevent riser failure. Typically, one or more flexible joints are placed intermediate the ends of the riser in order to increase riser flexibility. Another approach generally followed has been to attach the upper end of the marine riser to the production vessel by means of a pivotal connection and to join the lower end of the riser to the ocean floor via a universal joint disposed on the ocean floor. The submerged flowlines are typically connected to the universal joint supported riser by a plurality of elastomer-dependent flexible components such as hoses and/or swivels. The use of either flexible joints in the riser or a universal joint on the ocean floor to support the riser allows the riser to move due to movement of the production and storage vessel, as well as from forces exerted upon the riser itself by wind, waves, and/or water currents.
However, certain disadvantages result from the use of such flexible joints in the riser and/or a universal joint on the ocean floor. For example, flexible risers are subject to failure in deep water because of overstress and fatigue, in that tension must be applied to the riser to prevent it from buckling and the tension which must be applied increases rapidly with depth. In addition, stress levels induced within the riser by currents and the like, also increase with depth, although at a somewhat lower rate than the tensile load. These increased stress levels coupled with the cyclic variation in stresses caused by vessel movement may lead to accelerated fatigue failure.
With respect to the use of a universal joint on the ocean floor, a major disadvantage is associated with the required size and structural strength for such a universal joint. Further, the use of such universal joints requires the use of elastomer-dependent flexible components which are more quickly subject to failure than metal components. When such components do fail, it is necessary to use an expensive underwater maintenance vehicle, such as a tethered maintenance vehicle, to service and/or replace such components, as well as to connect such components to the marine riser in the first instance.
Accordingly, prior to the development of the present invention, there has been no production riser assembly for providing fluid communication between at least one flowline submerged in a body of water and a fluid storage means disposed adjacent the upper surface of the body of water, which does not require: flexible joints disposed along the length of the riser pipe; the use of a large and structurally strong universal joint on the ocean floor; the use of elastomer-dependent flexible components, such as hoses and/or swivels to connect the submerged flowline to the riser; and an expensive maintenance vehicle to install, service, and/or repair such elastomer-dependent flexible components.
Therefore, the art has sought a production riser assembly for providing fluid communication between at least one flowline submerged in a body of water and a fluid storage means disposed adjacent the upper surface of the body of water which does not require: a flexible joint in the marine riser; the use of a large universal joint on the ocean floor; the use of elastomer-dependent flexible components to connect the submerged flowline to the marine riser; and the use of an expensive maintenance vehicle to repair, install, and/or service the connections between the submerged flowline and the marine riser.